1. Field of the Invention
This invention relates to an improved vehicle automatic transmission control system.
2. Description of the Prior Art
The gear ratio (position) of a vehicle automatic transmission is generally selected by retrieval from a predetermined shift diagram stored in a microcomputer memory as a map, hereinafter referred to as "shift diagram map", using the throttle opening and the vehicle speed as address data. (See, for example, Japanese Laid-open Patent Publication No. 1(1989)-98743.)
Under some circumstances, the temperature of the ATF (automatic transmission fluid) of the transmission may become intolerably high. Since allowing the torque converter to continue to slip in such a condition leads to further increase in the ATF temperature, the prior art practice has been to cope with the situation by forcing the torque converter into lock-up mode so as to prevent slipping of the torque converter. However, since the torque converter cannot provide a torque increase during lock-up, the drivability of the vehicle is apt to be degraded owing to insufficient motive power during hill-climbing and the like.
Another drivability problem is experienced with automatic transmissions when driving on a road winding through hilly country. On entering a curve, the driver ordinarily releases the accelerator pedal and depresses the brake pedal. On coming out of the curve, he or she depresses the accelerator pedal again. When a vehicle with a prior art automatic transmission is driven in this way, the transmission shifts up when the accelerator pedal is released at the beginning of the curve and shifts down again when the road straightens out. The frequent shifting this leads to on a winding road annoys most drivers.
In addition, the engine speed has to be reduced for protecting the engine when the coolant temperature rises too high.
The assignee earlier proposed fuzzy logic-based automatic transmission control systems for overcoming the drawbacks of the conventional gearshift control based on a shift scheduling map in Japanese Laid-open Patent Publications No. 2(1990)-3739 and No. 2(1990)-85563 (also filed in the United States to mature as U.S. Pat. No. 5,036,730 and filed in EPO under 89306192.9); No. 2(1990)-3738 (also filed in the United States to mature as U.S. Pat. No. 5,079,705 and filed in EPO under 89306167.1); No. 2(1990)-138,558 and No. 2(1990)-138,561 (also filed in the United States to mature as U.S. Pat. No. 5,067,374 and filed in EPO under 89311976.8); No. 2(1990)-138,559, No. 2(1990)-138,560 and No. 2(1990)-150,558 (also filed in the United States to mature as U.S. Pat. No. 5,079,704 and filed in EPO under 89311970.1); and No. 4(1992)-8,964 (also filed the United States under U.S. Ser. No. 691,066 and EPO under 91303878.2).